The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, an automatic transmission achieving more speed stages has been developed to enhance fuel economy and improve drivability.
Such an automatic transmission achieving more speed stages allows to improve power performance and driving efficiency when downsizing an engine. Providing high efficiency multiple-speeds transmissions with smooth step ratios that can be used as an index closely related to drivability such as acceleration before and after a shift increases competitiveness of an automatic transmission in the automotive field.
However, in the automatic transmission, as the number of speed stages increase, the number of internal components increase, and as a result, mountability, cost, weight, transmission efficiency, and the like may still deteriorate.
Accordingly, development of a planetary gear train that improves efficiency with a small number of components may be provided in order to increase a fuel efficiency enhancement effect through the multiple-speeds.
An eight-speed automatic transmission has been introduced recently and a planetary gear train for an automatic transmission enabling more speed stages has been developed.
However, since a conventional eight-speed automatic transmission has gear ratio span of 6.5-7.5 (gear ratio span is an important factor for securing linearity of step ratios), improvement of power performance and fuel economy may be minimal.
In addition, if an eight-speed automatic transmission has a gear ratio span larger than 9.0, we have also found that it is difficult to secure linearity of step ratios. Therefore, driving efficiency of an engine and drivability of a vehicle may be deteriorated, and thus, development of high efficiency automatic transmissions which achieve at least nine forward speed stages is desired.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.